1. Field of the Invention
The present invention relates to a coil-embedded dust core, which may be used in inductors having a unitary structure with a magnetic core and in other electronic components. The present invention also relates to a method for manufacturing the coil-embedded dust core.
2. Description of the Related Art
In recent years, electric and electronic equipment has become more compact, and dust cores that are compact (low in height) yet able to accommodate large current have come to be in demand.
Materials used for dust cores are ferrite powder and ferromagnetic metal powder, but ferromagnetic metal powder has larger saturation magnetic flux density than ferrite powder and its DC bias characteristics may be maintained even in a strong magnetic field. Consequently, in making a dust core that can accommodate large current, using ferromagnetic metal powder as a material for dust core has become mainstream.
In addition, in order to further the effort to make the core more compact (lower in height), a coil body in which a coil and compacted magnetic powder form a unitary structure has been proposed. In the present specification, an inductor having such a structure may be called a “coil-embedded dust core.”
A manufacturing method for a surface-mount type inductor having a structure of a coil-embedded dust core has been proposed in the past. For example, Japanese Patent Laid-Open No. 5-291046 discloses that an exterior electrode is connected to an insulation-coated conduction wire, and these are enclosed in magnetic power, which is then compressed into a magnetic body. In this case, connection parts are inside the magnetic body, which makes them prone to failures while compressing. In the present specification, a “connection part” refers to a part where components are electrically connected to each other, and a part where a component is connected to an external electrode is called a “terminal section.”
Japanese Patent Laid-Open No. 11-273980 discloses a method of compression-forming flat powder and a coil using a binder. As an example of the publication, there is disclosed, the steps of making a composite material using a Fe—Al—Si metal alloy powder with an aspect ratio of approximately 20 and a silicone resin as an insulating material, and compression-forming the composite material together with a coil. However, no consideration has been given to connection parts between the coil and terminal sections, and joint failures (including poor joining) are likely to occur due to the fact that joining is difficult since it takes place between the magnetic body section and an electrode at the interface with the core.
Furthermore, a method of manufacturing an inductor using ferrite as a magnetic material is disclosed in Japanese Patent No. 2958807. Here again, part of the terminal that forms a connection part with the coil is inside the core, which makes it prone to failures in the connection parts during the process to form a unitary structure. Japanese Patent No. 3108931 describes a method of manufacturing an inductor by compression-forming while having a coil and a terminal section vertically interposed in a green body. Failures are likely to occur in the connection parts in this case as well.
As stated above, a coil-embedded dust core has a structure in which large inductance can be obtained in spite of its small size. However, as electric and electronic equipment becomes rapidly more compact, the demand for improved quality of coil-embedded dust core is growing. Specifically, there are demands to prevent joint failures between a coil and terminal sections; to prevent insulation failures of a coil and terminal sections with respect to magnetic powder; to make components even more compact; and to have larger inductance.
The coil-embedded dust core or the inductor proposed in the above-described Japanese Patent Laid-Open Nos. 5-291046 and 11-273980, Japanese Patent Nos. 2958807 and 3108931 can be improved in terms of quality. Namely, the coil-embedded dust core or the inductor in Japanese Patent-Laid-Open Nos. 5-291046 and 11-273980, and Japanese Patent No. 2958807 has a coil and terminal sections embedded within magnetic powder, which makes it prone to joint failures between the coil and the terminal sections or insulation failures of the coil and the terminal sections with respect to the magnetic powder. When a joint failure or an insulation failure occurs, it is difficult to determine the cause of the failure and in many cases takes a long time, since the coil and the terminal sections form connection parts inside the magnetic powder.
Furthermore, the inductor described in Japanese Patent No. 3108931 entails a high possibility for a joint failure to occur in connection parts between a coil and terminal sections after molding, due to the fact that a dust core is made using a coil that already has connection parts formed with terminal sections. When a joint failure occurs in a connection part, determining the cause is difficult and time-consuming.